This invention relates to a method of controlling the air-fuel ratio of an air-fuel mixture being supplied to an internal combustion engine, and more particularly to a method of this kind applied to an internal combustion engine having its cylinders divided into a plurality of cylinder groups, which is adapted to control the individual air-fuel ratios of mixtures being supplied to respective ones of the cylinder groups, independently of each other.
An air-fuel ratio feedback control method for an internal combustion engine has been proposed, e.g. by Japanese Provisional Patent Publication No. 57-188743, in which the concentration of a particular ingredient, e.g. oxygen, contained in exhaust gases emitted from the engine is detected by an oxygen concentration sensor (hereinafter referred to as "the O.sub.2 sensor") arranged in the exhaust system of the engine, and when the engine is operating in a normal operating condition, the air-fuel ratio is controlled in closed loop or feedback mode in response to a signal indicative of the O.sub.2 concentration from the O.sub.2 sensor, to a predetermined value, e.g. a theoretical air/fuel ratio (this manner of controlling the air-fuel ratio is hereinafter called "the O.sub.2 feedback control), to thereby reduce fuel consumption and improve emission characteristics of the engine.
Another method of controlling the air-fuel ratio has been proposed, e.g. by Japanese Provisional Patent Publication No. 58-217749, in which when the engine is operating in one of particular operating regions (e.g. a high load operating region, and a mixture-leaning region), the O.sub.2 feedback control is interrupted, and the air-fuel ratio is controlled in open loop mode to one of predetermined values corresponding to the above one particular operating region of the engine, which is best suited for the one particular operating region.
A further air-fuel ratio feedback control method has been proposed, for instance, by Japanese Provisional Patent Publication No. 58-101242, for a multicylinder internal combustion engine such as a V-type engine, which has a plurality of (e.g. six) cylinders divided into a plurality of (e.g. two) groups each of which comprises three cylinders, for example, and is connected with respective one of a plurality of divided exhaust passage portions, wherein a plurality of O.sub.2 sensors are arranged in respective ones of the exhaust passage portions, and the air-fuel ratios of mixtures being supplied to respective ones of the cylinder groups are controlled in a feedback manner responsive to the output values from corresponding ones of the O.sub.2 sensors, independently of each other.
However, according to the last-mentioned proposed method, the determination as to whether the engine is operating in a condition wherein the O.sub.2 feedback control should be effected or in a condition wherein the open loop mode control should be effected is made with respect to each of the cylinder groups independently of each other. This can result in determination that the cylinder groups are operating in different conditons from each other. In such a case, while the air-fuel ratio of a mixture or mixtures being supplied to one or some of the cylinder groups is controlled in the O.sub.2 feedback mode to a value equal to the theoretical air-fuel ratio, the air-fuel ratio of the other mixture(s) being supplied to the other cylinder group(s) is controlled in open loop mode to a value or values richer or leaner than the theoretical air-fuel ratio, resulting in deterioration of the driveability of the engine. Particularly, when the engine is operating in a predetermined high load operating region wherein the air-fuel ratio should be controlled in open loop mode to achieve a richer air-fuel ratio, if part of the cylinder groups is supplied with a mixture of which the air-fuel ratio is controlled in the O.sub.2 feedback mode to the theoretical air-fuel ratio, a required output torque of the engine cannot be obtained, thus deteriorating the driveability of the engine to a great extent.